Here is the conclusion of Dan Kay's blog series "Filler Metal Makes Braze Paste." Parts 1-3 can be found here.
Potential Oxidation Problems
Please be aware that the finer the mesh size of the brazing powder used in your furnace brazing, the greater the risk of oxidation of that powder. This depends on the quality (dryness) of your furnace atmosphere if brazing in a regular atmosphere furnace or the leak-up rate of your furnace when vacuum brazing. Figure 4 should help you understand why this is the case.
If you were to fill a container with -140 mesh powder and then fill an identically sized container with -325 mesh powder, and then each box was dumped out onto separate large sheets and you added up the surface areas of each powder particle on each of the two sheets, which group of powder particles would have a greater cumulative total surface area (the -140 mesh powder group or the -325 mesh powder group)? The answer is the total cumulative surface area of all the -325 mesh powder particles will be far, far greater than the cumulative surface area of all the -140 mesh powder particles.
Therefore, since there will be much greater surface area exposed to the furnace atmosphere when using smaller and smaller mesh size powders, it can be understood that you might not be able to effectively braze with -325 mesh powder in a marginal furnace atmosphere (an atmosphere with too high a content of oxygen or moisture) because of the much greater total amount of surface area exposed to potential oxidation when compared to using -140 mesh powder.
We asked our subject matter experts if there were any specific technologies or products that their partners were demanding training for, and whether it has changed over the past year or so. Here’s what they said:
A number of company websites incorrectly state that mesh size represents the number of openings per square inch of the wire-mesh screen. This is NOT true. Mesh size is the number of openings per linear inch in that screen.
Per American Society of Testing and Materials (ASTM) specification E11 “Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves,” sieve size designations represent the number of openings per linear inch.
The National Institute of Standards and Technology (NIST) special Publication 960-1 “Particle Size Characterization” states (p. 28), “Identification of sieves by their sieve number refers to the number of wires per linear inch forming that sieve.”
The American Society for Materials (ASM), in its “Glossary of Metallurgical Terms and Definitions” (from ASM Metals Reference Book, 3rd Edition) defines “mesh” as: “(1) The number of screen openings per linear inch of screen; also called mesh size.”
If a company has a vacuum furnace with a poor leak-up rate (e.g., 40 microns per hour leak rate) or if their atmosphere furnace was operating with a dew point of about -40°F (-40°C) or wetter, they may find it difficult to braze with a fine mesh powder (such as -325 mesh powder) since that fine mesh powder exposes far more surface area to all the oxygen and moisture in that marginal atmosphere than would a coarser powder.
I’ve seen fine mesh powders ball up in a marginal atmosphere and not flow out properly, whereas a coarser powder in the same environment flowed out because it had far less surface area of BFM powder exposed to the oxygen in that marginal atmosphere.
Atmosphere QC Test for Your Furnace
Put two different mesh-size powders of the same BFM alloy on a sheet (keep the two small piles of BFM powder well separated from each other), and run them in one of your regular brazing production runs. Do NOT spread the powders too thinly on these test coupons. Place them in small piles on each sheet. Then, after the brazing cycle, compare the flowing characteristics of the two small piles of powder.
They both should have flowed out nicely. If the -325 mesh powder tends to ball up on the sheet while the -140 mesh powder flows out, however, then your furnace atmosphere is becoming marginally poor. This test can be a quick way to catch that before it actually hurts some of your production parts.